Cubing a Cylinder: Cube Roots by Proportional Rectangles
A large rectangle divided into proportional parts to find the cube roots of 1 through 8
This dense mathematical sheet develops a geometric method for cubing a cylinder and for reducing several cylinders to a single cylinder of square cross-section. A large rectangle n b r g is divided into eight proportional sub-rectangles, and straight lines drawn from a marginal point f cut off segments whose lengths give the cube roots of 1 through 8 (labelled root of 8, root of 3, root of 2, root of 1). The surrounding text works through the ratios — an eighth, a quarter, and so on — that relate the distances of these roots from the face n b, while a large ruled construction fills the lower half of the sheet.
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Cubing a cylinder by drawn construction
Drawing the line f o tangent to the corner n of the cylinder, at a distance along its length equal to the face n b one finds the cube root of eight, which is two (o v and v L); drawing f r tangent to the corner of the eighth-part a b p g yields the cube root of one eighth, which is one, so that parallelepiped is cubed.
Roots of 8, 3, 2 and 1 labelling the rectangle
The proportional rectangle is annotated with the cube roots root of 8, root of 3, root of 2 and root of 1, accompanied by a series of point-letters (r v n x, t, S, r, e, d, K c and so on) that mark the successive divisions.
Proportional eighths of the face n b
The reasoning steps through the eighths: a b is 1/8 of n b, c b is 2/8, d b is 3/8, e b is 4/8, and so on, with n b in turn an eighth of b g; because the face n b and the length b g are each divided into eighths, the eighths contain equal quantity whichever way the cylinder is cut.
r n is seven-eighths of r g
A short marginal note to the left of the figure states the proportion r n is 7/8 of r g.
